sh: Disable generic IRQ probing.

IRQ autoprobing hasn't actually worked for us at all since very early in
2.6, but no one seems to have noticed given that none of the drivers
that use it see much testing.

yenta_socket is the odd one out, and that depends on PCI IRQs which are
fixed on all SH platforms anyways. Consequently, turning off autoprobing
fixes up crashes triggered by yenta_socket and at least gets it working
again on r7785rp.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>